Providing drive signals for a differential drive modulator

ABSTRACT

An optical waveguide structure forms an MZI in proximity to an electro-optic material. A first (second) electrical input port is configured to receive a first (second) drive signal. The second drive signal has a negative amplitude relative to the first drive signal. A first (second) transmission line is configured to propagate a first (second) electromagnetic wave over at least a portion of a first (second) optical waveguide arm to apply an optical phase modulation. A drive signal interconnection structure is configured to provide a first electrical connection between the first electrical input port and an electrode shared by the transmission lines, and a second electrical connection between the second electrical input port and respective electrodes of the transmission lines; and is configured to preserve relative phase shifts between the drive signals. Input impedances at the first and second electrical input ports are substantially equal to each other.

TECHNICAL FIELD

This disclosure relates to providing drive signals for a differential drive modulator.

BACKGROUND

A modulator is generally used to modulate a provided carrier wave with data to provide a modulated data signal. For example, an optical wave can be modulated using an optical modulator in an optical communication system. One type of optical modulator uses a Mach-Zehnder interferometer (MZI) to modulate the amplitude of an optical wave that is split at an input over two arms (e.g., waveguides) of the MZI and combined with respective phase shifts at an output, which is also known as a Mach-Zehnder modulator (MZM). Some MZMs are travelling-wave MZMs (TW-MZMs), which apply phase shifts over the arms using an electrical wave that propagates over a transmission line to apply the phase shifts. An example of a device that use can use electrical fields (static or traveling-wave) to modulate a refractive index in, or in proximity to, the waveguide arms of an MZM is a device that includes an electro-optic (E-O) material (e.g., an E-O crystal) within the modulator structure. Such a modulator may be known as an E-O crystal MZM. Various challenges may be encountered when attempting to operate such MZMs at relatively high frequencies (e.g., frequencies above 40 GHz), including challenges associated with drivers that provide drive signals (e.g., differential drive signals) for controlling the E-O phase shifts applied to the arm(s) of the MZM.

SUMMARY

In one aspect, in general, an apparatus comprises: an electro-optic material; an optical waveguide structure forming a Mach-Zehnder interferometer in proximity to the electro-optic material, where the Mach-Zehnder interferometer includes a first optical waveguide arm and a second optical waveguide arm; a first electrical input port configured to receive a first drive signal; a second electrical input port configured to receive a second drive signal that has a negative amplitude relative to the first drive signal; a first transmission line configured to propagate a first electromagnetic wave over at least a portion of the first optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the first optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; a second transmission line configured to propagate a second electromagnetic wave over at least a portion of the second optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the second optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; and a drive signal interconnection structure configured to provide a first electrical connection between the first electrical input port and an electrode shared by the first and second transmission lines, and a second electrical connection between the second electrical input port and respective electrodes of the first and second transmission lines, where the drive signal interconnection structure is configured to preserve relative phase shifts between the first and second drive signals; where an input impedance at the first electrical input port and an input impedance at the second electrical input port are substantially equal to each other.

Aspects can include one or more of the following features.

The first transmission line is terminated with a structure that has an impedance that is substantially equal to a characteristic impedance of the first transmission line, and the second transmission line is terminated with a structure that has an impedance that is substantially equal to a characteristic impedance of the second transmission line.

The first transmission line and the second transmission line are each terminated with a structure that has a resistance that is substantially equal to a real part of a differential impedance associated with the first transmission line and the second transmission.

The apparatus further comprises an impedance transforming structure configured to transform an electrical impedance associated with at least one of the first electrical connection or the second electrical connection such that an input impedance at the first electrical input port and an input impedance at the second electrical input port are substantially equal to each other.

The drive signal interconnection structure comprises a first wire-bond between the first electrical input port and the electrode shared by the first and second transmission lines, and a second wire-bond between the second electrical input port and a metal trace that is contiguous with each of the respective electrodes of the first and second transmission lines.

Lengths of the first and second wire-bonds are different from each other and selected to preserve relative phase shifts between the first and second drive signals.

The drive signal interconnection structure comprises a first metal trace at a first layer of an integrated circuit in electrical contact with the first electrical input port and the electrode shared by the first and second transmission lines, and a second metal trace at a second layer of the integrated circuit contiguous with each of the respective electrodes of the first and second transmission lines.

A length of the first metal trace is substantially equal to lengths of respective arms of the second metal trace.

The drive signal interconnection structure comprises a first wire-bond between the first electrical input port and the electrode shared by the first and second transmission lines, a second wire-bond between the second electrical input port and an electrode of the first transmission line, and a third wire-bond between the electrode of the first transmission line and an electrode of the second transmission line.

The third-wire bond is bonded at an angle selected to preserve relative phase shifts between the first and second drive signals.

The drive signal interconnection structure comprises a first wire-bond between the first electrical input port and the electrode shared by the first and second transmission lines, a second wire-bond between the second electrical input port and an electrode of the first transmission line, a plurality of wire-bonds between the electrode of the first transmission line and an electrode of the second transmission line.

The electrode shared by the first and second transmission lines and the electrode of the first transmission line each include a delay element configured to provide a time delay relative to the electrode of the second transmission line.

In another aspect, in general, an apparatus for providing drive signals for controlling a modulator comprising a first electrical input port for receiving a first drive signal and second and third electrical input ports for receiving a second drive signal that has a negative amplitude relative to the first drive signal comprises: circuitry configured to provide a first electrical signal; circuitry configured to provide a second electrical signal and a third electrical signal, where the second electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the third electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the second electrical signal has a negative voltage amplitude relative to the first electrical signal, and the third electrical signal has a negative voltage amplitude relative to the first electrical signal; a first output port configured to provide the first electrical signal to the first electrical input port; a second output port configured to provide the second electrical signal to the second electrical input port; and a third output port configured to provide the third electrical signal to the third electrical input port.

Aspects can include one or more of the following features.

The apparatus further comprises the modulator, where the modulator comprises at least one electro-optic travelling wave Mach-Zehnder modulator.

The apparatus further comprises the modulator, where the modulator comprises an optical waveguide structure forming a Mach-Zehnder interferometer in proximity to electro-optic material, where the Mach-Zehnder interferometer includes a first optical waveguide arm and a second optical waveguide arm; a first transmission line configured to propagate a first electromagnetic wave over at least a portion of the first optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the first optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; a second transmission line configured to propagate a second electromagnetic wave over at least a portion of the second optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the second optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; and a first wire-bond between the first electrical input port and an electrode shared by the first and second transmission lines, a second wire-bond between the second electrical input port and an electrode of the first transmission line, a third wire-bond between the third electrical input port and an electrode of the second transmission line, and a plurality of wire-bonds between the electrode of the first transmission line and the electrode of the second transmission line.

The circuitry configured to provide the second electrical signal and the third electrical signal comprises signal splitting circuitry at an output of a differential-output driver circuit that comprises two output wires providing the first drive signal and second drive signal, respectively.

The splitting circuitry includes one or more delay elements to preserve relative phase shifts among the first electrical signal, second electrical signal, and third electrical signal.

The circuitry configured to provide the second electrical signal and the third electrical signal comprises transistor-level circuitry within a driver circuit that comprises three output wires providing the first electrical signal, second electrical signal, and third electrical signal, respectively.

The transistor-level circuitry comprises at least one of a back terminated current mode driver, an open collector current mode driver, or a double emitter follower push pull (DEFPP) voltage mode driver.

In another aspect, in general, a method for providing drive signals for controlling a modulator comprising a first electrical input port for receiving a first drive signal and second and third electrical input ports for receiving a second drive signal that has a negative amplitude relative to the first drive signal comprises: providing a first electrical signal; providing a second electrical signal and a third electrical signal, where the second electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the third electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the second electrical signal has a negative voltage amplitude relative to the first electrical signal, and the third electrical signal has a negative voltage amplitude relative to the first electrical signal; providing, from a first output port, the first electrical signal to the first electrical input port; providing, from a second output port, the second electrical signal to the second electrical input port; and providing, from a third output port, the third electrical signal to the third electrical input port.

Aspects can have one or more of the following advantages.

Some of the techniques for providing and/or applying drive signals to modulators (e.g., MZMs) are able to provide differential drive signals in a manner that is able to operate effectively at high frequencies (e.g., greater than 40 GHz, such as around 100 GHz or higher). For example, a “push-pull” differential signal includes two signals that are complementary to each other: a first signal S (also called the S complement, or simply S) and second signal S (also called the S complement, or simply S), which has a negative amplitude relative to S, such that the relationship S=—S is at least approximately maintained during operation of the modulator. So, the sign of the amplitude of S is the opposite of the sign of the amplitude of S, and the magnitude (i.e., absolute value) of the amplitude of S is substantially equal to the magnitude of the amplitude of S. Given the symmetry between the complementary signals, the designation of which one is labeled as S, and which one is labeled as S, can be interchanged in any of the examples described and illustrated herein. These differential drive signals generally enable better signal integrity and reduced power consumption compared to modulators that are controlled by a single-ended driver, especially for high-frequency operation.

Some of the approaches described herein show techniques for implementing a 3-port driver that is configured for driving a differential drive modulator, and some of the approaches described herein show techniques for interconnecting a differential driver (e.g., a 2-port driver, or a 3-port driver) to an MZM in ways that preserve signal integrity (e.g., amplitudes and relative phases). The approaches apply to transmission line implementations that use a S-S-S electrode configuration with two S electrodes on both sides of a center S electrode, or a G-S-S-S-G electrode configuration that additionally include ground electrodes on both sides, as described in more detail below. Also described herein is an example of a passive termination circuit that can be used at the end of the transmission line electrodes to properly bias the driver output signals for a driver with an open-collector (OC) architecture.

Other potential benefits of differential (also called “balanced”) drive signals over single-ended drive signals include lower even-order harmonic generation at the driver output, and a more confined electric field, which reduces potential crosstalk, especially in circumstances where the driver and modulator are integrated on the same die. Enhancements to signal integrity of the drive signals delivered to a TW-MZM include increased signal-to-noise ratio (SNR) and therefore increased achievable propagation distance and/or modulation format complexity (and higher throughput). For example, a larger dynamic swing in the drive signals increases SNR through an increased optical modulation amplitude (OMA). There is also a higher average optical power transmitted in the MZM.

For the same total voltage swing there is a lower total TW-MZM dynamic power consumption, for example, in some cases by up to a factor of two. Also, since the required peak-to-peak voltage V_(pp) is split between a signal pair, each drive signal (S and S) requires half the amplitude as compared to a single-ended drive signal. The voltage needed to achieve a relative phase shift of 180 degrees in the MZM (V_(π)˜0.6-1.5 V) potentially enables the direct use of CMOS inverters (with a lower swing) to drive the MZM, without necessarily requiring the use of an external driver. This advantages potentially provide significant savings of cost, module footprint, and/or energy.

Other features and advantages will become apparent from the following description, and from the figures and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to-scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity.

FIGS. 1A and 1D are schematic diagrams of an example modulator driven by 2-port and 3-port drivers, respectively.

FIGS. 1B and 1C are schematic diagrams of example implementations of the modulator of FIG. 1A.

FIGS. 2A and 2B are schematic diagrams of example 3-port drivers.

FIGS. 3A, 3B, and 3C are schematic diagrams of example optoelectronic circuit configurations.

FIGS. 4A and 4B are schematic diagrams of example driver and modulator devices.

FIGS. 5A, 5B, and 5C are schematic diagrams of example driver and modulator devices.

FIGS. 6A and 6B are schematic diagrams of example driver and modulator devices.

FIGS. 7A, 7B, and 7C are circuit diagrams of example driver circuits.

FIG. 8 is a schematic diagram of an example bias circuit.

DETAILED DESCRIPTION

Differential-output drivers can be advantageous over single-end output drivers because they allow features such as: (1) cancellation or strong suppression of common-mode noise and other common impairments (like even-order harmonics) that may be present at a DAC/driver channel output, 2) an increase in the available swing at the modulator, and/or 3) for a fixed swing and fixed modulator/termination resistance, up to half the dynamic power consumption at the TW-MZM termination. The latter can be achieved because, for a prescribed optical phase shift at the TW-MZM, differential-driving allows half the swing of each drive signal compared to single-ended driving. This allows for a theoretical reduction by up to a factor of two in dynamic driver power consumption at the radio frequency (rf) termination of the TW-MZM compared to the single-ended case

${P_{SE} \approx \frac{V_{{rms}^{2}}}{Z_{0}}},$

since

${{2x\frac{\left( {V_{rms}/2} \right)^{2}}{Z_{0}}} = {\frac{1}{2}\frac{V_{{rms}^{2}}}{Z_{0}}}},$

where Z₀ is the characteristic impedance. Furthermore, differential-output drivers can also reduce the static power consumption by reducing the supply voltage since the swing is shared between two complements of a differential pair.

A differential driver reduces the supply voltage by half the swing, while still maintaining the same headroom as single-ended driver. Additionally, differential driving also allows for unterminated topologies such as Emitter Follower Push Pull (EFPP) and Open-Collector (OC), whereas single-ended driving are in some cases restricted to a traveling-wave amplifier (TWA) or a terminated single-ended driver.

In the context of TW-MZMs, drivers with differential channel outputs can be used in some cases to drive transmission lines that use coplanar strip (CPS) or dual coplanar waveguide (CPW) rf electrode configurations such as the following:

-   -   a) S-S (CPS, ‘series push-pull’ MZM architecture for SiPhot),     -   b) G-S-S-G (dual-CPS), or     -   c) G-S-G-S-G (dual-CPW, fully shielded).

These transmission line electrode configurations can be fabricated using integrated modulator technologies such as InP and SiPhot, which respectively rely on the quantum-confined Stark effect (QCSE) and plasma dispersion (free-carrier refraction). However, for linear-phase electro-optic (E-O) crystals such as bulk LiNbO₃ (LN), or integrated platforms such as a thin-film of LiNbO₃ or Silicon-Organic Hybrid (SOH) on a substrate (e.g., on SiO₂), due to the relatively low linear capacitance (i.e., absence of pn junction loading in the optical waveguides) and the field-effect nature of the modulators, electrode configurations of the type G-S-G (for single-ended driving), and S-S-S or G-S-S-S-G (for differential driving) provide advantages.

Some of the described approaches are directed to achieving optimal interconnection between a differential driver and a modulator that has a radio frequency (rf) transmission line electrode configuration of the type S-S-S or G-S-S-S-G (e.g., as shown in FIGS. 1A-1D). In some of the examples described herein, the S-S-S electrode configuration is used to show implementations of a differential drive electrode configuration for an E-O crystal TW-MZM, but the techniques described herein can also be applied to G-S-S-S-G electrode configuration, and other integrated modulator technologies that may benefit from S-S-S or G-S-S-S-G differential electrode configurations, such as SOH modulators. In some implementations, the S signal from a 2-port driver is split on-chip in interconnection circuitry that is fabricated on, or bonded to, the same die as the MZM. In other implementations, a 3-port driver provides an S signal and two S signals at respective driver output ports, and the S signal is split within the driver (e.g., as shown in FIG. 2A), and in some implementations, at the transistor level (e.g., as shown in FIG. 2B). In some implementations, a passive termination circuit can be used to properly bias the driver output signals for an open-collector (OC) architecture. In either the 2-port or 3-port driver implementations, the connections between the driver and the modulator can be formed using any of a variety of techniques, including wire-bonds, or a controlled collapse chip connection, also called flip-chip or C4 connection. Also, the output ports of the driver may serve as input ports of the TW-MZM, including any wire-bonds or other structures extending to various portions of the TW-MZM.

Referring to FIG. 1A, a modulator 100A includes an electro-optic material 102. For example, the electro-optic material 102 can be an electro-optic crystal, or any material (e.g., polymers and/or SOH) that produces an optical index change in response to an electric field (e.g., when being electrically poled). An optical waveguide structure 104 forms a Mach-Zehnder interferometer in proximity to the electro-optic material 102 (e.g., with waveguide cores formed in ridges etched into the electro-optic material 102, or deposited or grown on the electro-optic material 102, or a slot optical waveguide with electro-optic material formed between higher index ridges, such as Si ridges, as shown in FIG. 3C). The Mach-Zehnder interferometer includes two optical waveguide arms as part of the optical waveguide structure 104. A first electrical input port 106A is configured to receive a first drive signal, and a second electrical input port 106B is configured to receive a second drive signal that has a negative amplitude relative to the first drive signal. A first transmission line 108A is configured to propagate a first electromagnetic wave over at least a portion of a first optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the first optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material 102. A second transmission line 108B is configured to propagate a second electromagnetic wave over at least a portion of a second optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the second optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material 102. A drive signal interconnection structure 110 is configured to provide a first electrical connection between the first electrical input 106A port and an electrode (not shown) shared by the first and second transmission lines 108A and 108B (e.g., as shown in FIGS. 1B and 1C), and a second electrical connection between the second electrical input port 106B and different respective electrodes of the first and second transmission lines 108A and 108B (i.e., different from the shared electrode). The drive signal interconnection structure 110 is configured to preserve relative phase shifts between the first and second drive signals. Generally, the drive signal interconnection structure 110 provides interconnection between a driver (i.e., any source of drive signals) and a MZM structure.

In some implementations, an input impedance at the first electrical input port 106A and an input impedance at the second electrical input port 106B are substantially equal to each other. For example, referring to FIG. 1B, a modulator 100B includes electrodes 114A, 114B, and 114C on the surface of the electro-optic material 102. One transmission line is formed between the center electrode 114B and one of the outer electrodes 114A, and another transmission line is formed between the center electrode 114B and the other outer electrode 114C. In this example, an impedance transforming structure 112 is configured to transform an electrical impedance associated with an electrical connection to the shared center electrode 114B such that an input impedance at the first electrical input port 106A and an input impedance at the second electrical input port 106B are substantially equal to each other, as will be described below in more detail. In this example, the transmission lines are also terminated with respective structures 116A and 116B that each has an impedance that is substantially equal to a characteristic impedance of the first transmission lines, which substantially avoids any reflections back into the transmission lines. Alternatively, referring to FIG. 1C, a modulator 100C includes multiple impedance transforming structures 113A and 113B to transform the electrical impedances to match the input impedances at the input ports 106A and 106B. Referring to FIG. 1D, a modulator 100D includes a drive signal interconnection structure 120 that is configured to include three separate input ports 122A, 122B, and 122C for connection to output ports of a 3-port driver that is configured to provide multiple versions of a complement signal that has a negative phase of a primary signal, as described in more detail below.

Referring to FIG. 2A, one example of a 3-port driver 200A includes a differential-output driver circuit 202 that provides an S signal and an S signal, and splitting circuitry 204 passes the S signal to an output port 206B, and provides two S signals at respective output ports 206A and 206C. Referring to FIG. 2B, another example of a 3-port driver 200B includes a differential-output driver circuit 212 that includes internal transistor-level circuitry 214 that provides an S signal and two S signals, with the S signal provided to an output port 216B, and two S signals provided to respective output ports 216A and 216C. In some implementations of both the drivers 200A and 200B, the two S signals each have a voltage magnitude approximately equal to the voltage magnitude of the S signal and a negative voltage amplitude compared to the S signal (i.e., opposite sign), and a current approximately half of the current of the S signal. Generally, the S signals each carry half the power of the S signal and each have opposite polarity with respect to the S signal. Optionally, in some implementations there may be one or more output ports (not shown) that are grounded (i.e., at zero voltage).

Referring to FIG. 3A, a side view of an optoelectronic circuit 300A (e.g., a photonic integrated circuit, using a silicon-on-insulator or other semiconductor based fabrication process) shows portions of a TW-MZM including a center electrode 302 and two outer electrodes 303 and 304, shown here with propagation axis normal to the page such that electric field lines between electrodes for a rf electrical waves propagating down the transmission lines are shown within the plane of the page. A first waveguide arm 306 formed as a rib optical waveguide from an electro-optic material 307 (e.g., a thin film of Lithium Niobate) has its index of refraction modulated by an electric field between electrodes 302 and 303, and a second waveguide arm 308 formed as a rib optical waveguide from the electro-optic material 307 has its index of refraction modulated by an electric field between electrodes 302 and 304. The electro-optic material 307 can be deposited over a layer of silicon dioxide 310 that is over a substrate 312, and additional silicon dioxide 310 can be deposited over these structures. Optionally, in some implementations, there are additional electrodes 314 and 315 on the outside of the TW-MZM structure that are grounded (i.e., at zero voltage).

Referring to FIG. 3B, another example of an optoelectronic circuit 300B additionally includes a second metal layer 320 that is used provide a connection between the outer electrodes 303 and 304 using metal vias 323 and 324, respectively, connected through the metal layer 320, as described in more detail below. Referring to FIG. 3C, another example of an optoelectronic circuit 300C uses slot optical waveguides 330 and 331 formed from a semiconductor material 332 (e.g., silicon), with an electro-optic material 334 filling and surrounding the slot optical waveguide 330, and an electro-optic material 335 in filling and surrounding the slot optical waveguide 331. The optoelectronic circuit 300C can be fabricated using a silicon-organic hybrid (SOH) fabrication technique, for example.

Some approaches for interconnecting a 2-port driver to a modulator are based on a symmetric delay-matching (also called skew-matching) technique for preserving the phases of the two S signals. FIGS. 4A and 4B show implementations for interconnecting driver signals from two output ports of a driver chip to input ports of an optoelectronic chip containing a TW-MZM. In these implementations, integrated metal traces split the S signal on the optoelectronic chip before they connect to electrodes that form transmission lines of the TW-MZM. Both implementations use a S-S-S electrode configuration. Also, in both implementations, the rf path lengths for the two S signals and the S signal are all substantially equal, from the output ports of the driver to the terminations of the transmission lines of the TW-MZM. Optionally, an impedance transforming structure (also called an “impedance buffer” or “Z buffer”) can be used to transform an electrical impedance associated with at least one of two electrical connections from the 2-port driver such that input impedances at the two electrical input ports of the optoelectronic chip, as seen by the output ports of the driver chip, are substantially equal to each other.

Referring to FIG. 4A, a driver chip 400 is connected to an optoelectronic chip 402A by a drive signal interconnection structure 404A that is formed by wire-bonds 406 and 408 and metal structures within a single metal layer of the optoelectronic chip 402A. A TW-MZM is formed on the optoelectronic chip 402A by structures that include a center electrode 410, outer electrodes 411 and 412, and a Mach-Zehnder interferometer with waveguide arms 413 and 414. The wire-bond 406 is formed between an S contact pad of the driver chip 400 and an outer electrode contact pad 415 in a metal layer of the optoelectronic chip 402A. A v-shaped metal trace connection 416 in the same metal layer connects to each of the outer electrodes 411 and 412. The wire-bond 408, which is longer than the wire-bond 406, is formed between an S contact pad of the driver chip 400 and a center electrode contact pad 418. The lengths of the wire-bonds 406 and 408 are selected so that the propagation delay across a wide bandwidth for rf signals between the driver and the beginning of the transmission lines of the TW-MZM are substantially equal. This propagation delay matching preserves relative phase shifts between the S and S signals. In some implementations, wedge-bonds can be installed to achieve relatively precise relative timing for matching the propagation delay. The driver chip 400 can also optionally include ground contacts (G) as shown in this example.

Between the contact pad 418 and the center electrode 410 is an impedance buffer 420, which compensates for an impedance difference between the impedance for the parallel combination of split S signals and the impedance of the single S signal at the input ports of the optoelectronic chip 402A, where the input ports are defined at the beginning of the wire-bonds 406 and 408, respectively. Alternatively, in other examples, there can be impedance buffers between the ends of the v-shaped metal trace connection 416 and the outer electrodes 411 and 412, instead of, or in addition to the location shown in this example. The change in impedance that is provided by the impedance buffer 420 can be configured based on various structural factors, including the exact coplanar waveguide geometry of the transmission lines, such as the width and/or spacing of the three metal traces that form the electrodes of the transmission lines. The impedance buffer 420 can be formed from one or more layers of resistive material, and/or conductive material, and can include circuit components such as resonant stubs or transformers. The components, materials and/or structural geometries can be configured to provide inductance and/or capacitance that may be needed to compensate for any impedance mismatch over a broad rf signal bandwidth. An impedance buffer 420 may not be needed in some implementations if the input impedances are matched based on the geometries of the electrodes 410, 411, and 412. For example, if the outer electrodes 411 and 412 are each half the width of the center electrode 410 so that their resistance is about twice as high, then their equivalent combined parallel impedance at the input port for the S signal may be substantially equal to the impedance at the input port for the S signal. Or, more generally, the outer electrodes 411 and 412 may have an equivalent parallel characteristic impedance Z₀ that is substantially equal to the characteristic impedance Z₀ of the center electrode 410.

At the end of each transmission line is a termination structure that has an impedance that is substantially equal to a characteristic impedance of that transmission line. In this example, the termination structures 422 and 423 have impedances that are substantially matched over the rf spectrum of drive signals that will be provided by the driver chip 400. For example, the material of the termination structures 422 and 423 can be a resistive material in the same layer as the metal of the electrodes 410, 411, and 412.

Referring to FIG. 4B, the driver chip 400 is connected to an optoelectronic chip 402B by a drive signal interconnection structure 404B that is formed by wire-bonds 426 and 428 and metal structures within two metal layers of the optoelectronic chip 402B. A TW-MZM is formed on the optoelectronic chip 402B by structures that include a center electrode 430, outer electrodes 431 and 432, and a Mach-Zehnder interferometer with waveguide arms 433 and 434. The wire-bond 426 is formed between an S contact pad of the driver chip 400 and an outer electrode contact pad 435 in a top metal layer of the optoelectronic chip 402B. The wire-bond 428 is formed between an S contact pad of the driver chip 400 and a center electrode contact pad 436 in the top metal layer of the optoelectronic chip 402B. A v-shaped metal trace connection 438 in the top metal layer connects to each of the outer electrodes 431 and 432. The center electrode contact pad 436 is electrically connected to the center electrode 430 by a metal trace 440 that is in a lower metal layer underneath the top metal layer, with appropriate vertical metal contacts or vias between the two metal layers at the ends of the metal trace 440. In this implementation, delays due to the v-shaped metal trace connection 438 and the delay due to the metal trace 440 are substantially equal since their lengths are substantially equal. So, the lengths and shapes of the wire-bonds 426 and 428 can be substantially identical to each other since they do not need to compensate for different delays downstream.

At the end of each transmission line is a termination structure that has an impedance that is substantially equal to a characteristic impedance of that transmission line. In this example, the termination structures 442 and 443 have impedances that are substantially matched over the rf spectrum of drive signals that will be provided by the driver chip 400.

Generally, in the interconnection structures 404A and 404B are compact and configured to reduce impairments. For example, the parasitic inductances associated with the metal traces are relatively small since the lengths of the metal traces are relatively short.

Some approaches for interconnecting a 2-port driver to a modulator are based on an asymmetric delay-matching technique for preserving the phases of the two S signals. FIGS. 5A and 5B show implementations for interconnecting driver signals from two output ports of a driver chip to input ports of an optoelectronic chip containing a TW-MZM. In these implementations, a wire-bond splits the S signal on the optoelectronic chip on a portion of the electrodes that form transmission lines of the TW-MZM. Both implementations use a S-S-S electrode configuration. Also, in both implementations, the rf path lengths for the two S signals and the S signal are all substantially equal, from the output ports of the driver to the terminations of the transmission lines of the TW-MZM. Optionally, one or more impedance transforming structures can also be used in these implementations, but are not explicitly shown.

Referring to FIG. 5A, a driver chip 500 is connected to an optoelectronic chip 502A by a drive signal interconnection structure 504A that is formed by wire-bonds 506, 507, and 508. A TW-MZM is formed on the optoelectronic chip 502A by structures that include a center electrode 510, outer electrodes 511 and 512, and a Mach-Zehnder interferometer with waveguide arms 513 and 514. The wire-bond 506 is formed between an S contact pad of the driver chip 500 and an outer electrode contact pad 515. The wire-bond 507 is formed between an S contact pad of the driver chip 400 and a center electrode contact pad 518. The wire-bond 508 connects a portion of the outer electrode 511 contiguous with the contact pad 515 to a portion of the outer electrode 512 on the other side of the center electrode 510. The wire-bond 508 is angled forward by an angle α to prevent what would be a relative time delay (also called rf skew) between the two S signals propagating along the outer electrodes 511 and 512. This wire-bond 508 is located over the unloaded section (i.e., the section before MZM waveguide arms) of the TW-MZM.

At the end of each transmission line is a termination structure that has an impedance that is substantially equal to a characteristic impedance of that transmission line. In this example, the termination structures 522 and 523 have impedances that are substantially matched over the rf spectrum of drive signals that will be provided by the driver chip 500.

Referring to FIG. 5B, the driver chip 500 is connected to an optoelectronic chip 502B by a drive signal interconnection structure 504B that is formed by wire-bonds 526, 527, and 528. A TW-MZM is formed on the optoelectronic chip 502B by structures that include a center electrode 530, outer electrodes 531 and 532, and a Mach-Zehnder interferometer with waveguide arms 533 and 534. The wire-bond 526 is formed between an S contact pad of the driver chip 500 and an outer electrode contact pad 535. The wire-bond 527 is formed between an S contact pad of the driver chip 400 and a center electrode contact pad 538. The wire-bond 528 connects a portion of the outer electrode 531 contiguous with the contact pad 535 to a portion of the outer electrode 532 on the other side of the center electrode 530. The wire-bond 528 is not angled forward in this implementation, but instead the relative time delay caused by the signal propagation time over the wire-bond 528 is compensated for by a delay element 539 built into a portion of the electrode 531 to remove any relative time delay between the two S signals propagating along the outer electrodes 531 and 532. There is also a delay element 540 built into a portion of the center electrode 530 to remove any relative time delay between the S signal and the two S signals. The delay elements 539 and 540 can be formed, for example, by an extra length of transmission line (e.g., extra curvature), or by local substrate/cladding removal as to adjust the microwave group index. Alternatively, a slow wave structure such as a sub-wavelength patterned periodic structure can be used. For example, FIG. 5C shows an example implementation of such periodic structure delay elements on the electrodes 530 and 531 provided by T-rails 541. This wire-bond 528 is also located over the unloaded section (i.e., the section before the MZM waveguide arms) of the TW-MZM.

At the end of each transmission line is a termination structure that has an impedance that is substantially equal to a characteristic impedance of that transmission line. In this example, the termination structures 542 and 543 have impedances that are substantially matched over the rf spectrum of drive signals that will be provided by the driver chip 400.

In both optoelectronic chips 502A and 502B, TW-MZM includes periodic wire-bonds 550 joining the two outer electrodes along the TW-MZM structure. Alternatively, in implementations fabricated using a back-end process that allows for multiple metal layers, the the periodic wire-bonds can be replaced by periodic crossings in a second metal layer, for example. Whether using wire-bonds or a second metal layer, these periodic conductive connections enable more uniform propagation of the S signals over the two outer electrodes in both optoelectronic chips 502A and 502B. Such, periodic conductive connections between the outer electrodes can also be included in the optoelectronic chips 402A and 402B.

Additionally, in some alternative implementations, an impedance buffer can be included at the start of the TWMZM in the optoelectronic chips 502A and 502B if the input impedances between the input port for the S signal pair and input port for the S signal are not substantially equal.

Other implementations are possible in which the signal interconnection structures are configured for skew matching such that the propagation delays for the S signal paths and the S signal path are substantially equal across the drive signal interconnection structure and the TW-MZM structure.

In some implementations, a driver chip, such as the driver chip 400 or the driver chip 500, can be configured for attachment using different fabrication techniques. For example, a controlled collapse chip connection, also called flip-chip or C4 connection (e.g., using solder bumps and copper pillars), can be used to connect the driver chip directly to the optoelectronic chip. In a flip-chip driver implementation, the longer wire-bond 408 in the implementation of FIG. 4A could be replaced by a delay within the driver chip 400 so that the S and S signals arrive at the same time at the portion of the TW-MZM where the modulation starts. Alternatively, a longer trace could be integrated in a substrate common to the driver chip 400 and the optoelectronic chip 402A.

Alternatively, in some implementations, a substrate or interposer chip can be used between the driver chip and the optoelectronic chip, in which case, any portion of the periodic conductive connections and/or delay-matching structures could be formed on the substrate or interposer without requiring use of wire-bonds. Alternatively, if a back-end process allows for additional redistribution layers (e.g., PI/Cu on top of the existing passivation) any portion of the periodic conductive connections and/or delay-matching structures could be formed through the redistribution layers.

Some approaches interconnect a 3-port driver to a modulator, where the modulator includes a contact pad for a single S signal and two separate contact pads for two different S signals. FIG. 6A shows a technique in which splitting of the S signal occurs within a driver chip 600A using signal splitting circuitry 602 at an output of a differential-output driver circuit 604 and prior to the output contact pads 606A, 606B, and 606C. This example enables impedance matching, delay matching, and impedance transformation close to the driver circuit 604, which can mitigate reflections and can be managed on a sub-micron scale within the driver chip 600A. The contact pads 606A, 606B, and 606C are connected (e.g., using wire-bonds) to corresponding contact pads on an optoelectronic chip 610. A TW-MZM is formed on the optoelectronic chip 610 by structures that include a center electrode 611, outer electrodes 612 and 613, and a Mach-Zehnder interferometer with waveguide arms 614 and 615.

FIG. 6B shows a technique in which splitting of the S signal occurs within a driver chip 600B using transistor-level circuitry within a differential-output driver circuit 620 that has three output wires connected directly to respective output contact pads 622A, 622B, and 622C. This example enables two versions of the S signal to be provided, and the transistor-level circuitry can be configured to be compatible with either voltage mode or current mode driver circuitry. The contact pads 622A, 622B, and 622C are connected (e.g., using wire-bonds) to corresponding contact pads on the optoelectronic chip 610.

FIGS. 7A, 7B, and 7C show example implementations of the differential-output driver circuit 620. Referring to FIG. 7A, a back terminated current mode driver configuration is shown, where the S signal is split using half sized transistors double the pull up resistor compared to the S signal. Referring to FIG. 7B, an open collector current mode driver configuration is shown, where the S signal is split using half sized transistors compared to the S signal. Referring to FIG. 7C, a double emitter follower push pull (DEFPP) voltage mode driver is shown, where the S signal is split using half sized transistors double the pull up resistor compared to the S signal. The configuration of the transistors and metal conducting paths within the circuits can be arranged differently in the integrated circuit that in the circuit diagrams shown. For example, in some implementations the transistor providing the S signal is positioned between transistors providing the different S signals.

FIG. 8 shows an example of a bias circuit 800 for terminating the transmission lines of a TW-MZM in an optoelectronic chip 802 driven by a driver chip 804 that is configured to use an open-collector driver topology/architecture. In this example, for clarity, the transmission line contacts of the TW-MZM are shown but the optical waveguide arms of the MZM are not shown. The bias circuit 800 can be used to provide RF output DC biasing for such a driver. In this example, the bias circuit 800 includes resistive elements shown in the illustrated circuit diagram as resistors having one of two resistor values 2N Ohms for the outer electrodes, and N Ohms for the center electrode. The value N is substantially equal to real part of the differential impedance of the TW-MZM Re{Z_(diff)}. The parallel capacitance for each outer electrode resistor shown in the illustrated circuit diagram as capacitors have a capacitance value C that can be determined according to an impedance matching criteria. For example, at low frequencies, the in-band capacitor impedance is selected such that Zc<<N, and the capacitance can also be set with consideration of the target low-frequency cutoff of the apparatus. The series inductance after the node combining the currents from the three paths shown in the illustrated circuit diagram as an inductor has an inductance value L that is high enough to be substantially equal to an ideal choke preventing RF power from propagating into the bias circuit 800. A voltage source 806, which can be connected from off-chip, provides a constant (DC) voltage for the bias circuit 800. The resistive elements, capacitive elements, and inductive element can be integrated into the optoelectronic chip 802 in some implementations. In some implementations, the relatively high values of C and/or L may be provided by adding discrete passive electronic components (e.g., small form-factor Si-capacitors). In implementations in which a driver does not require RF output DC biasing (e.g., not an open-collector topology/architecture), some of the resistive elements (e.g., resistors other than the center electrode resistor), capacitive elements, and/or inductive element can be omitted.

From a DC perspective, this TW-MZM termination bias scheme works for either a S signal split on the optoelectronic chip, or on the driver chip. The DC voltage drop is the same on all three termination resistors ensuring consistent RF output DC biasing. For a split on the optoelectronic chip, driven by a 2-port driver, each pathway for the S signal receives the same bias current (e.g., half the current flow in the top and bottom MZM electrodes with respect to the center electrode, and then add up before reaching the S driver output). For a split within a 3-port driver, each path from the ports providing each of the two different versions of the S signal receives half the bias current with respect to the port providing the S signal (e.g., as in FIG. 6A or FIG. 6B). From a RF perspective, each rf travelling wave will effectively see N Ohms at the terminated end of the transmission line.

While the disclosure has been described in connection with certain embodiments, it is to be understood that the disclosure is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law. 

What is claimed is:
 1. An apparatus comprising: an electro-optic material; an optical waveguide structure forming a Mach-Zehnder interferometer in proximity to the electro-optic material, where the Mach-Zehnder interferometer includes a first optical waveguide arm and a second optical waveguide arm; a first electrical input port configured to receive a first drive signal; a second electrical input port configured to receive a second drive signal that has a negative amplitude relative to the first drive signal; a first transmission line configured to propagate a first electromagnetic wave over at least a portion of the first optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the first optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; a second transmission line configured to propagate a second electromagnetic wave over at least a portion of the second optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the second optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; and a drive signal interconnection structure configured to provide a first electrical connection between the first electrical input port and an electrode shared by the first and second transmission lines, and a second electrical connection between the second electrical input port and respective electrodes of the first and second transmission lines, where the drive signal interconnection structure is configured to preserve relative phase shifts between the first and second drive signals; where an input impedance at the first electrical input port and an input impedance at the second electrical input port are substantially equal to each other.
 2. The apparatus of claim 1, where the first transmission line is terminated with a structure that has an impedance that is substantially equal to a characteristic impedance of the first transmission line, and the second transmission line is terminated with a structure that has an impedance that is substantially equal to a characteristic impedance of the second transmission line.
 3. The apparatus of claim 1, where the first transmission line and the second transmission line are each terminated with a structure that has a resistance that is substantially equal to a real part of a differential impedance associated with the first transmission line and the second transmission.
 4. The apparatus of claim 1, further comprising an impedance transforming structure configured to transform an electrical impedance associated with at least one of the first electrical connection or the second electrical connection such that an input impedance at the first electrical input port and an input impedance at the second electrical input port are substantially equal to each other.
 5. The apparatus of claim 1, where the drive signal interconnection structure comprises a first wire-bond between the first electrical input port and the electrode shared by the first and second transmission lines, and a second wire-bond between the second electrical input port and a metal trace that is contiguous with each of the respective electrodes of the first and second transmission lines.
 6. The apparatus of claim 5, where lengths of the first and second wire-bonds are different from each other and selected to preserve relative phase shifts between the first and second drive signals.
 7. The apparatus of claim 1, where the drive signal interconnection structure comprises a first metal trace at a first layer of an integrated circuit in electrical contact with the first electrical input port and the electrode shared by the first and second transmission lines, and a second metal trace at a second layer of the integrated circuit contiguous with each of the respective electrodes of the first and second transmission lines.
 8. The apparatus of claim 7, where a length of the first metal trace is substantially equal to lengths of respective arms of the second metal trace.
 9. The apparatus of claim 1, where the drive signal interconnection structure comprises a first wire-bond between the first electrical input port and the electrode shared by the first and second transmission lines, a second wire-bond between the second electrical input port and an electrode of the first transmission line, and a third wire-bond between the electrode of the first transmission line and an electrode of the second transmission line.
 10. The apparatus of claim 9, where the third-wire bond is bonded at an angle selected to preserve relative phase shifts between the first and second drive signals.
 11. The apparatus of claim 1, where the drive signal interconnection structure comprises a first wire-bond between the first electrical input port and the electrode shared by the first and second transmission lines, a second wire-bond between the second electrical input port and an electrode of the first transmission line, a plurality of wire-bonds between the electrode of the first transmission line and an electrode of the second transmission line.
 12. The apparatus of claim 11, where the electrode shared by the first and second transmission lines and the electrode of the first transmission line each include a delay element configured to provide a time delay relative to the electrode of the second transmission line.
 13. An apparatus for providing drive signals for controlling a modulator comprising a first electrical input port for receiving a first drive signal and second and third electrical input ports for receiving a second drive signal that has a negative amplitude relative to the first drive signal, the apparatus comprising: circuitry configured to provide a first electrical signal; circuitry configured to provide a second electrical signal and a third electrical signal, where the second electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the third electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the second electrical signal has a negative voltage amplitude relative to the first electrical signal, and the third electrical signal has a negative voltage amplitude relative to the first electrical signal; a first output port configured to provide the first electrical signal to the first electrical input port; a second output port configured to provide the second electrical signal to the second electrical input port; and a third output port configured to provide the third electrical signal to the third electrical input port.
 14. The apparatus of claim 13, further comprising the modulator, where the modulator comprises at least one electro-optic travelling wave Mach-Zehnder modulator.
 15. The apparatus of claim 13, further comprising the modulator, where the modulator comprises an optical waveguide structure forming a Mach-Zehnder interferometer in proximity to electro-optic material, where the Mach-Zehnder interferometer includes a first optical waveguide arm and a second optical waveguide arm; a first transmission line configured to propagate a first electromagnetic wave over at least a portion of the first optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the first optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; a second transmission line configured to propagate a second electromagnetic wave over at least a portion of the second optical waveguide arm to apply an optical phase modulation to an optical wave propagating in the second optical waveguide arm according to an electro-optic modulation of a portion of the electro-optic material; and a first wire-bond between the first electrical input port and an electrode shared by the first and second transmission lines, a second wire-bond between the second electrical input port and an electrode of the first transmission line, a third wire-bond between the third electrical input port and an electrode of the second transmission line, and a plurality of wire-bonds between the electrode of the first transmission line and the electrode of the second transmission line.
 16. The apparatus of claim 13, where the circuitry configured to provide the second electrical signal and the third electrical signal comprises signal splitting circuitry at an output of a differential-output driver circuit that comprises two output wires providing the first drive signal and second drive signal, respectively.
 17. The apparatus of claim 16, where the splitting circuitry includes one or more delay elements to preserve relative phase shifts among the first electrical signal, second electrical signal, and third electrical signal.
 18. The apparatus of claim 13, where the circuitry configured to provide the second electrical signal and the third electrical signal comprises transistor-level circuitry within a driver circuit that comprises three output wires providing the first electrical signal, second electrical signal, and third electrical signal, respectively.
 19. The apparatus of claim 18, where the transistor-level circuitry comprises at least one of a back terminated current mode driver, an open collector current mode driver, or a double emitter follower push pull (DEFPP) voltage mode driver.
 20. A method for providing drive signals for controlling a modulator comprising a first electrical input port for receiving a first drive signal and second and third electrical input ports for receiving a second drive signal that has a negative amplitude relative to the first drive signal, the method comprising: providing a first electrical signal; providing a second electrical signal and a third electrical signal, where the second electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the third electrical signal has a voltage magnitude approximately equal to the voltage magnitude of the first electrical signal and a current approximately half of the current of the first electrical signal, the second electrical signal has a negative voltage amplitude relative to the first electrical signal, and the third electrical signal has a negative voltage amplitude relative to the first electrical signal; providing, from a first output port, the first electrical signal to the first electrical input port; providing, from a second output port, the second electrical signal to the second electrical input port; and providing, from a third output port, the third electrical signal to the third electrical input port. 